The purpose of this investigation is to study the nature of host resistance, with particular reference to the role of interferon and interferon production by leukocytes and macrophages of the reticuloendothelial system. The contribution of interferon production by phagocytic cell elements is being studied both in in vitro models and in experimental infections in vivo. Other studies are directed toward the definition of factors which may modify host resistance to viral infections, and the development of experimental models which may permit the delineation of the role of antibody and interferon in host resistance.